The 3GPP Long Term Evolution (LTE) a set of standards in the mobile network technology tree providing a set of enhancements to the Universal Mobile Telecommunications System (UMTS), while adopting 4G mobile communication technology, including an all-IP flat networking architecture. The LTE systems are capable of downlink peak rates of at least 100 Mbps, an uplink of at least 50 Mbps and supports scalable carrier bandwidths, from 1.4 MHz to 20 MHz using both frequency-division duplexing (FDD) and time-division duplexing (TDD). The main advantages with LTE are high throughput, low latency, plug and play, FDD and TDD in the same platform, an improved end-user experience and a simple architecture resulting in low operating costs.
A generic setup in a mobile radio communication system 100 (which can be an LTE system) is illustrated in FIG. 1. In the system 100, base stations such as, 105, 110, 115, serve user devices (such as, 120), specifically, the user devices located in an area (cell, marked with dashed line in FIG. 1) surrounding a respective base station. Here, the base station 110 serves the user device 120.
The communication between a base station and a user terminal is usually synchronized to occur at predetermined time slots. Since the user devices may be mobile, they may move from an area of one base station to an area of a neighboring base station. For example, the user device 120 in FIG. 1, may have previously been served by the base station 105, and has recently moved from the cell where it was served by the base station 105, to the cell where it is served by the base station 110. In this case (i.e., when entering a new cell), as well as when a user device initiates connecting to the radio communication system 100, there is a procedure involving a message exchange between the user device and the base station, for establishing and synchronizing the communication there-between.
FIGS. 2 and 3 illustrate the procedure for establishing and synchronizing the communication between a user device and a base station (e.g., the base station 110 and the user device 120 in FIG. 1). FIG. 2 illustrates the messages exchanged for establishing and synchronizing the communication. FIG. 3 illustrates the timing of these messages. First, the user device 120 acquires a signal 111 broadcasted by the base station 110 and indicating time slots and frames (i.e., the Physical Random Access Channel—PRACH) useable for sending uplink messages (i.e., from user devices to the base station) including, for example, messages with connection requests. A delay occurs between when the base station 110 sends the signal 111 and when the user device 120 receives the signal 111 due to the travel time of the signal 111 between the base station 110 and the user device 120. However, the user device 120 does not have the information that would enable correcting for this delay, and, thus, the user device 120 sends a signal 112 including a RACH preamble, at one of the time slots learned from the base station 110 and assuming no delay. Such a signal is, in fact, a request for connecting to the radio communication system via the base station.
The base station 110 receiving the signal 112 from the user device 120 is capable to estimate the time correction that the user device 120 user has to make in order to achieve a true synchronization with the base station for uplink (from the user device 120 to the base station 110) traffic. Further, the base station 110 sends a signal 113 directed to the user device 120 according to the user identifier included in the signal 112, and indicating the time correction so that later uplink messages (e.g., 114) are synchronized.
As illustrated in FIG. 4, the RACH preamble includes a cyclic prefix (RACH CP) portion lasting TCP and a sequential portion TSEQ. The above described aspects of radio communication systems are described in 3GPP TS documents, current versions of which are incorporated herewith by reference. In particular, pertinent aspects are defined and described in 3GPP TS 36.211, 3GPP TR 21.905, 3GPP TS 36.201, 3GPP TS 36.212, 3GPP TS 36.213, 3GPP TS 36.214, 3GPP TS 36.104, 3GPP TS 36.101 and 3GPP TS 36.321.
Thus, using the signal 112 that includes the RACH preamble, the base station 110 is able to identify the user device 120 and to determine the round trip delay (i.e., the time correction). Upon receiving a response message 113 the user device is enabled to send synchronized messages. Once synchronization is achieved, the user device 120 is enabled to send synchronized messages to the base station 110, while sharing physical uplink channels with other users.
A conventional manner of processing received signals is illustrated in FIG. 5. The boxes in FIG. 5 correspond to steps of a method. Some of these steps may be executed on the same processor, but each box may represent different physical devices. In other words, the boxes in FIG. 5 correspond to software, hardware or a combination thereof.
The top row in FIG. 5 represents a normal processing for messages received via the physical uplink channel. In a first data processing portion 150, the cyclical portion (symbol CP) of a received signal is removed 152 and, then, the signal is subjected to a one-half subcarrier frequency shift 154. In radio communication systems other than LTE, the one-half subcarrier frequency shift may not be necessary. The signal is then divided in time pieces corresponding to a fraction (e.g., 1/14 or 1/12) of a millisecond (ms), these pieces being named symbols. An FFT per symbol is performed on each symbol (lasting 1/12 or 1/14 of the signal, and using 2048 points for 20 MHz bandwidth) at 156, before transmitting the resulting frequency domain signal pieces to an uplink processing module 160.
When the signal received at the base station is an unsynchronized signal including a RACH preamble (such as, signal 112 in FIG. 2), a super FFT is performed at (or in the module) 170 for about 1 ms of the signal, using as many as 24576 points for the whole signal bandwidth (e.g., 20 MHz). This super FFT involves a large amount of data to transport and buffer and requires a large amount of computation. Normal traffic processing (e.g., in the first data processing unit 150, etc.) may proceed in parallel to the data processing related to the RACH preamble. Depending on the amount of other data, the performance of both RACH preamble and normal traffic processing are impacted by the resources used for the super FFT.
The output of the super FFT is then processed in a second data processing portion 180 that is configured to receive a frequency domain signal to process and use the received signal for identifying and determining the time correction of the user device. Specifically, the second data processing portion 180 includes a module 182 selecting the 839 RACH subcarriers (as described, for example, in 3GPP TS 36.211, section 5.7.2) from the frequency domain signal output, a module 184 extracting a temporary identifier of the user device based on correlating the selected signal with Zadoff-Chu sequences, and, then, a module 186 performing an inverse FFT using 2048 points. The resulting time domain signal is then forwarded to a RACH detect module 190 for further detection. The 839 RACH subcarriers correspond to a frequency band of about 1 MHz width, but the RACH subcarriers frequency band does not have a fixed position within the whole signal bandwidth (e.g., 20 MHz). Never-the-less, most (e.g., 19 MHz) bandwidth of the whole (e.g., 20 MHz) bandwidth, is discarded in module 182.
The super FFT 170 is a substantial burden for the base station in terms of storage space and power, while most of its result is discarded immediately thereafter. Accordingly, it would be desirable to provide devices, systems and methods that process the uplink unsynchronized signals including the RACH preamble more efficiently than by performing the super FFT.